H-Field Shaping Using a Shorting Loop

ABSTRACT

In one embodiment the present invention includes a radio frequency identification (RFID) system with a shorting loop. The shorting loop at least partially surrounds the antenna. The shorting loop distorts the electromagnetic field generated by the antenna to improve the definition of the border of the read region of the antenna. In this manner, the RFID system provides more accurate discrimination between RFID tags inside the read region versus RFID tags outside the read region (i.e., improves the accuracy of determining that a particular RFID tag is inside the read region).

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional Application No. 61/046,658 filed Apr. 21, 2008 and titled “H-Field Shaping Using a Shorting Loop”, which is incorporated herein by reference.

BACKGROUND

The present invention relates to radio frequency identification (RFID) systems, and in particular, to RFID systems that operate in a constrained environment.

Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.

Detection (that is, detecting the presence or absence of an RFID tag) is an important issue with RFID systems. Strategies for improving detection include increasing tag power (e.g. with batteries), selecting frequencies that minimize “shadows”, increasing reader power, and increasing the number of readers. All these strategies are directed toward providing overlapping coverage areas to increase the likelihood that the RFID tag is detected.

Discrimination (that is, discriminating the location of the RFID tag) is another important issue. However, discrimination is somewhat in conflict with detection: For detection, overlapping coverage areas are good, but overlapping coverage areas make it harder to determine exactly which reader is closest to the tag. One strategy for improving discrimination is to increase the distance (separation) between readers.

The conflict between detection and discrimination is keenly felt in constrained environments where a number of readers are in close proximity, and it is desired to determine which reader is reading a particular tag. One example of such a constrained environment is a casino gaming environment in which the gaming tokens include RFID tags. As a specific example, a gaming table may have multiple locations that gaming tokens may be placed, and it is desirable both to detect (the ID of the token) and to discriminate (that the token is placed in a particular functional area of the gaming table).

The technical challenge is to insure that only tags inside a betting zone are read. An ideal method is that only tags inside a betting zone receive sufficient energy to respond and tags outside remain unpowered. A second method to differentiate signals coming from tokens inside a betting zone versus those tokens outside a betting zone is using a signal strength threshold. Signals above the threshold are believed to be inside the betting zone, while signals below the threshold are believed to be outside the betting zone. A third method, comparing the signal strength of an individual token signal as measured by two adjacent (or nearby) readers, can also be used. Cross-talk occurs when one either does not read a tag inside the betting zone or the betting zone erroneously reads a tag that is outside the betting zone. It remains desirable that the signal strength from tokens at the top of a stack inside the betting zone be greater than the signal strength of any tokens outside the betting zone.

FIG. 12 shows three views of a typical loop antenna 1204. The top view shows a three-dimensional view of the flux lines 1200 and the loop antenna 1204. The middle view shows a cut away side view along Section A-A of the top view. The bottom view shows a graph of the field strength 1202 as one traverses across Section A-A of the loop antenna 1204. Two features are noteworthy: the “null” 1210 and the “tail” 1212. The “null” 1210 is a region of low sensitivity arising from horizontal flux lines 1200 in the h-field that do not intersect the horizontal antenna loop 1204. Ideally, the null 1210 is outside the defined borders of the betting zone. If the “null” falls inside the betting zone, there will be “dead spots” in the betting zone that may result in read errors. The “tail” 1212 is a region of undesirable sensitivity outside the defined betting zone. If the “tail” is sufficient to energize tokens outside the betting zone, the result may be cross-talk errors.

Thus, there is a need for RFID systems that operate in a constrained environment.

SUMMARY

Embodiments of the present invention improve the definition of a border area between a desired read area and the area outside of the desired read area. In one embodiment the present invention includes a radio frequency identification (RFID) system. The RFID system includes an antenna and a shorting loop. The antenna generates an electromagnetic field that includes a magnetic field component. The shorting loop at least partially surrounds the antenna and distorts the magnetic field component to improve a definition of a border of a read region related to the antenna.

Other features may be included in other embodiments. The shorting loop may be displaced from the antenna by a gap; a size of the gap is selected to balance a read range and a discrimination factor. (The discrimination factor may correspond to a signal strength difference between a first RFID tag inside the read region and a second RFID tag outside the read region. The gap may be a constant distance between the shorting loop and the antenna. The gap may also be variable gap.)

The RFID system may include an RFID reader, coupled to the antenna, that provides power to the antenna to generate the electromagnetic field, that provides a first modulated signal to the antenna, and that receives a second modulated signal from the antenna. The first modulated signal communicates information to an RFID tag, and the second modulated signal communicates information from the RFID tag.

The RFID system may include an RFID reader, coupled to the antenna, that includes for reading a first RFID tag inside the read region and that excludes from reading a second RFID tag outside the read region. This determination may be made according to a low (if any) detected field strength of the second RFID tag.

The antenna may be a circular antenna, the shorting loop may be a circular shorting loop, and the circular shorting loop may surround the circular antenna. The antenna may be a rectangular antenna, the shorting loop may be a rectangular shorting loop, and the rectangular shorting loop may surround the rectangular antenna. The shorting loop may be made of a copper material. The shorting loop may constrain the electromagnetic field outside of the read region. A first signal strength of a first RFID tag within the border may be more than 10 dB greater than a second signal strength of a second RFID tag that is outside the border and adjacent to the first RFID tag.

The RFID system may include a plurality of RFID tokens that each include a magnetically permeable core that distorts the magnetic field component and, in accordance therewith, improves a field strength of the magnetic field component within the read region.

According to an embodiment, a method of performing radio frequency identification (RFID), includes providing a shorting loop that at least partially surrounds an antenna. The method further includes generating, with the antenna, an electromagnetic field that includes a magnetic field component. The method further includes distorting, with the shorting loop, the magnetic field component to improve a definition of a border of a read region related to the antenna. The method further includes reading an RFID tag in the read region.

According to an embodiment, a radio frequency identification (RFID) system includes a plurality of antennas and a plurality of shorting loops. Each antenna generates an electromagnetic field that includes a magnetic field component; the plurality of antennas includes a first antenna and a second antenna that is near the first antenna. Each shorting loop at least partially surrounds a corresponding one of the antennas; a first shorting loop of the plurality of shorting loops distorts the magnetic field component of the first antenna to reduce interference with the magnetic field component of the second antenna.

The following detailed description and accompanying drawings provide a better understanding of the nature and advantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D show an RFID gaming system 100 according to an embodiment of the present invention.

FIGS. 2A, 2B and 2C show various aspects of the performance of embodiments of the present invention.

FIG. 3 is a graph showing a comparison between an existing air core RFID tag and a ferrite core RFID tag according to an embodiment of the present invention.

FIG. 4 is a graph of a waveform showing a basic modulation scheme used by an embodiment of the present invention.

FIG. 5 is a graph of a waveform showing a typical command-response sequence according to an embodiment of the present invention.

FIG. 6 is a block diagram of a gaming table according to an embodiment of the present invention.

FIG. 7 is a block diagram of a motherboard according to an embodiment of the present invention.

FIG. 8 is a block diagram of a daughter board according to an embodiment of the present invention.

FIG. 9 is a block diagram of an antenna according to an embodiment of the present invention.

FIG. 10A is a top view (cut away) and FIG. 10B is a bottom view (cut away) of a token according to an embodiment of the present invention.

FIG. 11 is a block diagram of a token according to an embodiment of the present invention.

FIG. 12 shows three views of a typical loop antenna.

FIG. 13 shows three views of a loop antenna and a shorting loop according to an embodiment of the present invention.

FIG. 14 shows three views of a loop antenna and a shorting loop according to an embodiment of the present invention.

FIG. 15 is a flow diagram of a method of performing RFID according to an embodiment of the present invention.

DETAILED DESCRIPTION

Described herein are techniques for improved radio frequency identification (RFID) reading. In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include modifications and equivalents of the features and concepts described herein.

An embodiment of the present invention is directed to an RFID system in a gaming environment. For ease of description, the gaming environment provides context for describing an embodiment. It is to be understood that the form factor of the RFID system may be adjusted, and that RFID systems embodying principles of the invention may be used in environments other than gaming environments.

An embodiment of the present invention includes a shorting loop. (The shorting loop may also be referred to as a shielding loop or a shield loop.) The shorting loop surrounds the reader antenna and distorts the magnetic field to improve a definition of (i.e., better define) a read region related to the reader antenna (e.g., to discriminate between inside or outside of the read region).

The remainder of this disclosure is organized as follows. First, a general description is provided that describes a number of gaming system embodiments that may include a shorting loop. The gaming system embodiments may contain features such as gaming tables, reader systems, and RFID gaming tokens. Second, various details of the gaming system embodiments are provided. These details may include features such as the communications protocols, the command protocols, and the ferrite core in the RFID gaming tokens. Further details of the gaming system embodiments may be seen in U.S. patent application Ser. No. 12/351,774 for “Enhancing the Efficiency of Energy Transfer to/from Passive ID Circuits Using Ferrite Cores” filed Jan. 9, 2009. Third, the details of the shorting loop are provided.

General Description

Although the description uses the term “ferrite”, this term is to be broadly interpreted to refer to any type of magnetically permeable material. In general, a material is magnetically permeable if its magnetic permeability is greater than that of air.

One feature of an embodiment is that the amount of energy that may be scavenged by each RFID tag from the excitation (reader) antenna is increased, as compared to many existing RFID systems. Another feature of an embodiment is that the efficiency of the energy transfer from the excitation (reader) antenna to the RFID tag is increased, as compared to many existing RFID systems. Another feature of an embodiment is that the efficiency of the data transfer from the excitation (reader) antenna to the RFID tag is increased, as compared to many existing RFID systems. Another feature of an embodiment is that the efficiency of the data transfer from the RFID tag back to the reader antenna is increased, as compared to many existing RFID systems. These increases in energy and data transfer may include one or more of the following features according to an embodiment: increased stack height of gaming tokens that can be read (resulting from increased read range); reduced read errors within a stack of gaming tokens; improved discrimination between tokens inside a “betting spot” and tokens outside the betting spot; and improved read times.

As further detailed below, a gaming token embodiment includes a RFID tag with a ferrite core that steers the magnetic flux emanating from an excitation source. In a gaming environment, where gaming tokens may be stacked in a column, the ferrite materials in the center of the token efficiently steer the magnetic flux up the stack to energize all the chips in the stack. The excitation source (reader antenna) may be embedded in the playing surface of the gaming table.

In contrast, typical RFID readers (using tags without a ferrite core; sometimes referred to as “air core” tags) radiate their energy broadly. This less efficient coupling between reader and tags is a significant factor in read range.

The presence of the ferrite core, according to an embodiment, may enable one or more of the following features as compared to air core tokens:

-   -   1. The increased efficiency of the coupling between the         excitation (reader) source and the passive gaming token extends         the read range of the “tag-reader” system.     -   2. The increased efficiency of the coupling between the         excitation source and the passive gaming token allows the system         designer to trade off read range in exchange for a gaming token         with a lower “Q” with little or no net negative impact on read         range. Assuming sufficient energy is available to power the RFID         tag, this lower Q can minimize sensitivity to the interaction         that results when multiple tags are in close proximity.     -   3. Sufficient energy can be transferred to the passive circuitry         in the token that a more powerful processor can be used to         achieve improved data rates, security, and error         detection/correction.     -   4. The increased efficiency of the coupling between the gaming         token and the excitation source can be used to achieve improved         data rates, security, and error detection/correction.     -   5. Controlling the shape of the h-field makes it easier to         discriminate between tokens that should be read (i.e., tokens         that are in a selected betting zone) from tokens that should not         be read (i.e., tokens that are not in the selected betting         zone). This will minimize read errors due to cross-talk between         adjacent betting zones.

FIGS. 1A, 1B, 1C and 1D show an RFID gaming system 100 according to an embodiment of the present invention.

FIG. 1A shows a number of gaming tokens 102 stacked within a shorting loop 104 that surrounds a betting spot 106. The betting spot 106 may be located on a gaming table (not shown) on which the gaming tokens 102 may be placed to make a bet. The gaming table may include a reader 107 that has an RFID antenna 105 and other RFID reader electronics (not shown) for reading an RFID tag located within each gaming token 102.

The shorting loop 104 constrains flux lines 108 generated by the gaming tokens 102 in response to electromagnetic energy emitted by the RFID reader. The flux lines 108 (also referred to as the h-field) are generated by the RFID reader 107. The gaming tokens 102 are energized by this field. This field can be modulated by either the reader 107 (for a data uplink to the tokens 102) or the gaming tokens 102 (for a data downlink to the reader 107).

FIG. 1B is a cut away side view showing more details of the gaming tokens 102. Each gaming token 102 includes a ferrite core 120 and an annular RFID tag 122.

FIG. 1C shows a cut-away top view of the gaming token 102. The ferrite core 120 can be seen, as well as a loop antenna 124 as a component of the RFID tag 122.

FIG. 1D shows a cross-sectional view of the gaming token 102. The ferrite core 120 can be seen, as well as the loop antenna 124 and the other electronics 126 of the RFID tag 122.

The RFID tag 122 may be magnetically coupled. That is, although the electromagnetic radiation involved in RFID applications includes both an electrical field (e-field) and a magnetic field (h-field), an embodiment of the present invention uses the magnetic field. Essentially, a transformer is formed, with one winding in the excitation antenna in the reader and multiple windings—one in each token. In many existing RFID systems, the transformer has an “air core”. This air core may be inefficient, but this inefficiency may be a necessary tradeoff when not knowing the location of the RFID tag. In contrast, embodiments of the present invention use a “ferrite core” to improve the performance of the transformer created by the excitation antenna and the token antenna(s) 124. The use of ferrite cores is particularly effective when the gaming tokens 102 are stacked and the ferrite cores 120 deform the magnetic flux component of the electromagnetic energy received from the reader. This deformation of the magnetic flux concentrates the flux lines through the ferrite cores 120. The concentrated flux lines through the core 120 couple the electromagnetic energy in a more efficient manner than in a token 102 that lacks the ferrite core 120. In effect, the ferrite cores 120 steer the flux field to improve performance, as more fully described below.

The token antenna 124 scavenges electromagnetic energy (for example, from the h-field) from the reader. The other electronics 126 rectifies the energy and uses the energy to drive a processor. Data transfer from reader to token can be superposed on the carrier using modulation. Similarly, the processor can modulate the carrier to perform data transfer from token to reader to identify the RFID tag 122. According to one embodiment, a 13.56 MHz carrier is used. The carrier frequency 13.56 MHz has been found to couple well to ferrites.

FIGS. 2A, 2B and 2C show various aspects of the performance of embodiments of the present invention.

FIG. 2A is a cut away side view showing two stacked gaming tokens 102. Each gaming token 102 has a ferrite core 120. The thickness of the gaming token 102 is T and the thickness of the ferrite core is t. The ratio t/T is referred to as the “% ferrite”.

FIG. 2B is a perspective view showing a number of gaming tokens 102 stacked at a stack height of 3 inches. This stack height is used to compare embodiments of the present invention to other techniques.

FIG. 2C is a graph showing the performance of various materials. The x-axis shows the “% ferrite” (see FIG. 2A and related discussion) and the y-axis shows the signal strength. (The “% ferrite” of 100% indicates that the thickness of the ferrite core 120 is the same as the thickness of the gaming token 102.) For the material “material2”, the line 202 shows the received signal strength of “dB” at 100%. Given line 202 as a baseline for the desired signal strength, other materials may be used to give different performance characteristics. For example, the material “material1” shown by the line 204 indicates a thinner ferrite core 120 can be used to give the same performance as “material2”. The material “material3” shown by the line 206 indicates a thicker ferrite core 120 is needed to give the same performance as “material2”. As can be seen, more ferrite as a percentage of the thickness of the gaming token 102 gives a stronger signal. In addition, different ferrite materials have different performance characteristics.

FIG. 3 is a graph showing a comparison between an existing air core RFID tag and a ferrite core RFID tag according to an embodiment of the present invention. The x-axis shows the stack height of a stack of gaming tokens 102 (see FIG. 1A) and the y-axis shows the signal strength of the signal received by the topmost gaming token in the stack. For an air core RFID tag, the line 300 has a signal strength dB0 at the stack height H0. This stack height H0 may correspond to approximately 2.5 inches in existing systems. For a ferrite core RFID tag, the line 302 has the signal strength dB0 at the stack height H*. The stack height H* is greater than the stack height H0. Thus, embodiments of the present invention allow reading at greater stack heights as compared to existing systems. Correspondingly, for a given stack height H0, the received signal strength of the line 302 is greater than that of the line 300. Thus, embodiments of the present invention provide improvement in read height as compared to existing systems. In effect, the multiple stacked tags create a rod of ferrite material that steers the magnetic flux field up through the stack.

As discussed above, many existing RFID tags have a relatively high Q. The Q is relatively high to increase sensitivity and read range. However, when multiple tags are in close proximity, they tend to interact. This interaction changes their resonant frequency of operation. As a result, the tags are not energized and/or data is not successfully exchanged.

According to an embodiment of the present invention, the antennas in the gaming tokens 102 have a relatively lower Q than those in existing RFID tags. Having a relatively lower Q may also be referred to as being de-tuned. De-tuning helps to address the problem of unwanted interactions between tags, but also severely limits sensitivity and hence the read range. In contrast to many existing air core RFID tags, embodiments of the proposed invention use ferrite cores 120 to compensate for this loss of read range. When these gaming tokens 102 are stacked, the ferrite cores 120 concentrate the h-field and steer the flux. This allows the use of de-tuned tags which are not compromised by the presence of adjacent tags. The decrease in sensitivity from de-tuning is offset by the focused h-field. The result: the ability to read multiple tags in close proximity while maintaining excellent read range. In contrast, many existing air core RFID tags either cannot be read in close proximity or suffer in read range.

As a specific example, the Q of many existing RFID tags is between 10 and 20, whereas the Q of the antennas in the gaming tokens 102 is close to zero (in fact, a designer may want to make the Q as close to zero as possible). Ideally, the antenna in the gaming token 102 may present itself as a pure resistive load. In practice, however, there is parasitic capacitance that contributes to a modest Q (typically less than 1). For the system designer, one notable factor that determines what Q is possible (to maximize read range) is how closely coupled the tags are—which is dependent on how closely spaced they are. According to an embodiment, tags with a Q less than 5.0 provide an acceptable read range. According to an embodiment, tags with a Q less than 1.0 provide an improved read range.

According to an embodiment, the antennas in the gaming tokens 102 are untuned and have a resonance frequency that is well above 13.56 MHz. The lack of a tuned antenna limits sensitivity and hence the read range. As with the low Q antenna embodiment discussed above, the ferrite cores 120 concentrate the h-field and steer the flux, allowing the use of untuned tags which are not compromised by the presence of adjacent tags.

Modulation Details

FIG. 4 is a graph of a waveform 500 showing a basic modulation scheme used by an embodiment of the present invention. The x-axis corresponds to time and the y-axis corresponds to power level. The power output is modulated at approximately 4 KHz with the amplitude shifting between 100% (equivalent to a digital 1) and 80% (equivalent to a digital 0). This selection in amplitudes is a balance between maximizing the energy available to the tag and insuring that the signal strength of the digital data is sufficient to detect. The 256 u-second frame 502 is the basic frame of communication from the tag to the reader. This frame was chosen to be long enough for the required command and data strings (see below for detailed description of command structure). The edges (e.g., 504 and 506) of the 256 u-second frame are used to synchronize the tags with the reader.

Other modulation schemes may be used according to other embodiments, as desired according to design requirements. For example, the modulation frequency may be adjusted. As another example, the power levels for “1” and “0” may be shifted. As another example, the size of the data frame may be adjusted.

Tag Bits

Generic RFID uses up to 96 bits for unique serial numbers. Unlike generic RFID, the serial numbers for a closed environment need not be so long. A shorter serial number can improve read cycle times and reduce error rates. Using an estimate of 1 million tokens used in each of 1,000 closed environments, it was deemed that a universe of 1 billion (approximately 2E30) unique serial numbers is sufficient for an embodiment. Allowing for the discard of undesirable numbers with poor DC balance or too many successive 1s or 0s, a base serial number of 31 bits was chosen in an embodiment.

CRC checks may be used to insure that data is not corrupted during transmission. In general, the more CRC bits, the more robust the error detection—but with diminishing returns. A 13 bit CRC was chosen as a reasonable compromise between robustness and overhead, according to an embodiment. The CRC may be calculated using a polynomial, such as 0X1909, according to an embodiment. In other embodiments, other error detection or error correction schemes may be used.

Downlink Details

FIG. 5 is a graph of a waveform 600 showing a typical command-response sequence according to an embodiment of the present invention. The basic command structure is one of “master-slave” with all commands initiated by the reader. A complete read cycle starts when the 13.56 MHz carrier signal is powered up and ends when all tokens have been read and the carrier powered down. The waveform 600 shows a typical command-response sequence with “down-link” (reader to tag) commands 602 and “up-link” (tag to reader) responses 604. The reader may use a READ command to read tags. Because of the statistical likelihood of collisions, a complete read cycle may require multiple READ commands to read all tokens.

According to an embodiment, a SLEEP command may also be implemented. The SLEEP command may be as disclosed in U.S. Provisional Application No. 61/031,270 for “Dynamic Power Absorption of a Loop Antenna for Passive RFID Tags”, filed Feb. 25, 2008.

READ Command Details

The READ command includes two components according to an embodiment. One component is the iteration number. Another component is the window number.

The purpose of the iteration number is to eliminate repeated collisions from the same tokens during subsequent READ commands. A process in the token firmware determines which bits of the unique token ID are used to “randomly” select a response window. Because the iteration number is different for each read command in a given cycle, the likelihood of two tokens repeatedly colliding is minimized. The iteration number increments from 0 to 9. A special iteration number of 11 may be used for diagnostic purposes.

As noted above, the function of the modified Aloha protocol is improved when the number of tags in the field can be estimated, constrained (e.g., by limiting the physical area that may be occupied by tags), or otherwise known. (According to an embodiment, the number of tags may be estimated as disclosed in U.S. Provisional Application No. 61/046,690 for “Estimating the Number of RFID Tags in the Field”, filed Apr. 21, 2008. One feature of the ferrite core is that the close coupling of the tokens to the antenna results in an antenna impedance that is proportional to the number of tokens. This allows the reader to measure the antenna impedance, estimate the number of tokens in the excitation field, and dynamically adjust the number of response windows by including this variable in the command structure.) This information can be used to set the number of data frames for the responses. Too many data frames results in longer read cycle times. Too few data frames results in excess collisions from multiple tokens responding within the same data frame. It has been determined that good results occur when the number of data frames is approximately ten times the number of tokens. Alternatively, the number of tags can be estimated by using a moderate number of windows and seeing how many collisions there are, and then adjusting the number of windows upward or downward accordingly.

TABLE 1A shows the structure of the 16-bit READ command, starting from the most significant bit (MSB).

TABLE 1A Bit 15 14 13-10 9-7 6-0 Function START bit READ Iteration # Window # STOP bits command Value 1 0 0-9 0-7 0

TABLE 1B shows the number of frames corresponding to each window number.

TABLE 1B Window # 0 1 2 3 4 5 6 7 # of 32 64 128 256 512 1024 2048 reserved frames Read time 8 msec 16 msec 32 msec 64 msec 128 msec 256 msec 512 msec n/a

Uplink Details

According to an embodiment, the up-link data rate is 4 u-seconds/bit, thereby allowing the entire token ID of 46 bits to fit inside a single 256 u-second frame. The 4 u-second rate corresponds to the clock of the microprocessor in the tag. According to an embodiment, the microprocessor in the tag is a PIC microcontroller (PIC10F206T-I/OT from Microchip Technology Inc., Chandler, Ariz.). The reader DSP (digital signal processor) over-samples this data, leveraging its much higher processing power. As shown in TABLE 2, the 46 bit structure consists of a start bit, a 31 bit token ID, a 13 bit CRC, and one stop bit.

TABLE 2 Bit 45 44-14 13-1 0 Function START bit Token ID CRC STOP bit Value 1 0 or 1 0 or 1 1

Read Cycle Example

Assume a stack of 30 tokens in the betting area. The reader will detect this load and assign a window number of either 3 or 4. Assuming the worst case (4), the corresponding number of frames is 512—or 128 m-seconds to process this initial READ command. Assume a 10% collision rate—resulting in 3 unread tokens. Putting 27 tokens to sleep requires an additional 35 m-seconds. A second READ command using a window number of 0 corresponds to 32 frames or 8 m-seconds. If no collisions are detected on this second READ command, the total elapsed time for the read cycle is 171 m-seconds.

According to an alternative embodiment, 44 m-seconds are used to put 27 tokens to sleep, resulting in a total elapsed time for the read cycle of 180 m-seconds.

The uplink signal is essentially amplitude modulated, however, the phase of the returned signal is unknown due to the phase shift from the reader to the tag and the tag to the reader. To address this unknown phase shift, the receiver mixes the signal down to baseband creating I and Q channels. These I and Q channels are processed to recover the data from the tag.

FIG. 6 is a block diagram of a gaming table 700 according to an embodiment of the present invention. In general, the embodiment of FIG. 6 may be referred to as a reader system. The gaming table 700 includes eight betting spots 702 a-702 h (collectively 702), eight antennas 704 a-704 h (collectively 704), eight shorting loops 706 a-706 h (collectively 706), eight daughter boards 708 a-708 h (collectively 708), a motherboard 710, and a control system 712. (Some of the description of these components, e.g., the betting spots, the antennas, and the shorting loops, may be repeated from the descriptions of components having similar names that were discussed in previous sections.) The gaming table 700 may be used in conjunction with the ferrite core gaming tokens 102.

The gaming table 700, according to an embodiment, may include one or more of the following attributes:

-   -   1. A 13.56 MHz carrier.     -   2. A “modified Aloha” protocol.     -   3. A 256 u-sec “data frame” that modulates the carrier.     -   4. A 16-bit data word for commands to the tokens (140 KHz data         rate or 7 u-seconds per bit). According to another embodiment,         the data rate is 125 KHz or 8 u-seconds per bit.     -   5. A 46-bit token ID data (500 KHz data rate or 2 u-seconds per         bit; 31 bits for ID plus 13 bits for error detection). According         to another embodiment, the data rate is 250 KHz or 4 u-seconds         per bit. According to yet another embodiment, a start bit and a         stop bit may also be transmitted.

The RF communication uses a 13.56 MHz carrier generated by the motherboard 710 and sent to each daughter board 708. 13.56 MHz was chosen for three reasons: (1) the ferrite cores are permeable at this frequency, (2) data rates are reasonable, (3) FCC frequency allocations for this type of application.

Even though the “ferrite core” concept may not strictly be an RFID technology, the energy and data exchange leverages a “modified Aloha” protocol common to many RFID systems. This protocol was selected because, when there is a good signal to noise ratio and a reasonable estimate of the number of tags in the field is known, it provides a fast read cycle. Characteristics of this modified Aloha protocol include:

-   -   1. A “master-slave” command structure with all commands         initiated by the reader.     -   2. A defined number of response “windows”. Ideally, the number         of windows balances the need to manage collisions (more windows         is better) with the desire for fast read cycle times (fewer         windows is better).

A complete read cycle begins by turning on the 13.56 MHz carrier signal to power up the tokens in the excitation field and ends when all tokens in the excitation field have been read. Once powered up, the tokens wait for a command. The DSPs on each daughter board 708 modulate the carrier to send commands and data to the tokens at a data rate that the tokens (with only modest processing power) can capture. The number generator process in each token “randomly” assigns a response window. According to an embodiment, the token processor uses the iteration and window size to determine which of the bits in the ID number are used to define a response window. The daughter boards 708 attempt to read the ID of any chips within the range of its antenna. An error detection scheme identifies any collisions. Tokens that are read successfully are put to sleep and the process is repeated. Once all token IDs have been read, the data is sent to the PC.

The token data rate is the fastest that its internal oscillator can drive (250 KHz according to an embodiment); the DSP has the processing horsepower to manage the higher data rate of the down-link from the tokens. This asymmetry in data rates (up-link vs. down-link) aligns with the volume of data to get good read cycle times (much less bandwidth is required for commands than for token IDs). Both the 16-bit commands and the 46-bit IDs fit into the 256 u-sec data frame.

The number and arrangement of the betting spots 702 may be varied depending upon the specifics of the game to be played. In general, a betting spot 702 may be placed anywhere on the table 700 near which a measurement of RFID tags is desired to be made. (“Near” is a relative term that may vary depending upon the specific features implemented in the system. With the specific example configuration described here, “near” produces acceptable read performance at a distance up to approximately 6 inches above the antenna 704.) More specifically, the arrangement of betting spots 702 on the table 700 corresponds to a blackjack game where the only desired information is the IDs of gaming tokens currently being wagered.

The number and arrangement of the antennas 704 generally correspond to the number and arrangement of the betting spots 702. For example, for a given play area, the size of and spacing between the antennas 704 can be adjusted according to a desired performance threshold, and the betting spots 702 conform to the locations of the antennas 704. According to an embodiment, the antenna 704 may be rectangular in shape and 2 inches by 4 inches in size. According to an embodiment, the antenna 704 may be circular in shape and 4 inches in diameter. Other closed geometries (triangular, rhomboidal, pentagonal, hexagonal, ovular, etc.) or combinations of closed geometries (semi-circular, combined semi-circular and semi-rectangular, etc.) or even irregular closed geometries may be used in other embodiments. According to an embodiment, the antenna 704 may be spaced from the shorting loop 706 by a gap of one-quarter inch. According to an embodiment, the antenna 704 may be constructed on a FR-4 (flame retardant) printed circuit board. According to an embodiment, the antenna 704 may have a thickness of 0.031 inches.

The motherboard 710 can interface to the control system 712 that may be configured to operate in either a local manner or in a remote manner. When the control system 712 is local, the motherboard 710 may be connected via a connection such as USB (universal serial bus). When the control system 712 is remote, the motherboard 710 may be connected via a LAN (local area network) connection such as Ethernet. The Ethernet connection allows the control system 712 to be remotely located to control one or more gaming tables 700 as part of a larger system to monitor fraud or reward loyalty. Alternatively, the USB connection allows the mother board 710 to interface to the local control system 712 to help run demos, de-bug prototypes, and/or integrate diverse systems into a common data format. The local control system 712 can also maintain a copy of the database of “valid” IDs to insure continuous play even in the event of a breakdown in the LAN.

FIG. 7 is a block diagram of the motherboard 710 according to an embodiment of the present invention. The motherboard 710 includes a power supply 802, two control system interfaces 804 a and 804 b, a controller 806, and eight daughter board interfaces 808 a-808 h (collectively 808).

The power supply 802 supplies power to the controller 806 and the control system interfaces 804 a and 804 b.

The controller 806 implements the functionality of the motherboard 710. These functions may include providing power to the daughter boards 708, generating the carrier frequency (e.g., 13.56 MHz) to drive the reader antennas, generating the clock (e.g., 4 KHz) that the daughter boards 708 use to define the command and data windows, formatting the decoded serial data received from the tokens, etc. The controller 806 may be implemented with a complex programmable logic device (CPLD) or other type of circuit structure. According to an embodiment, the controller 806 may be implemented with a XC95144XL-10TQG100C CPLD from Xilinx, Inc., San Jose, Calif.

The control system interface 804 a interfaces the motherboard 710 with the control system 712. According to an embodiment, the control system 712 connects to the motherboard 710 in USB format and the controller 806 communicates in serial format, so the control system interface 804 a implements a USB-to-serial interface. The control system interface 804 a may be implemented by a FT232RL device from Future Technology Devices International Ltd., Glasgow, United Kingdom. According to an embodiment, the control system 712 connects to the motherboard 710 in Ethernet format, so the control system interface 804 b implements an Ethernet connection. The control system interface 804 b may be implemented by a LPC2368FBD100-S microcontroller from NXP Semiconductors, Eindhoven, The Netherlands.

The daughter board interfaces 808 provide connections between the motherboard 710 and the daughter boards 708. The system designer may choose to add or reduce the number of daughter boards 708 depending on the number of betting spots and the speed at which they must be read. According to an embodiment, one daughter board 708 drives one antenna 704. According to an embodiment, one daughter board 708 drives multiple antennas 704, and the read signals are multiplexed. The number of simultaneous reads will impact the required power.

FIG. 8 is a block diagram of the daughter board 708 a according to an embodiment of the present invention. The daughter board 708 a includes a transmitter 902, a receiver 904, a directional coupler 906, and an antenna switch 908.

The transmitter 902 includes an amp supply 920, an amplitude modulator 922, an amplifier 924, and a bandpass filter 926. The transmitter 902 receives three signals from the motherboard: a power signal PWR_PWM, an amplitude modulated data signal AM_DATA, and the 13.56 MHz carrier signal. The transmitter 902 modulates the data signal onto the carrier signal and provides the modulated carrier signal to the directional coupler 906.

The directional coupler 906 provides the modulated carrier signal to the antenna switch 908, which provides the modulated carrier signal to the antenna 704 for transmission. (According to an embodiment, the daughter board 708 a includes two antenna connectors and can drive two antennas. The antenna switch 908 determines which of the antennas are used.) The tags scavenge the transmitted energy and, in response, further modulate the signal with their ID information, which the directional coupler 906 provides to the receiver 904.

The receiver 904 includes mixers 930 a and 930 b, low pass filters 932 a and 932 b, amplifiers 936 a and 936 b, differential amplifiers 938 a and 938 b, analog to digital converters 940 a and 940 b, a programmable logic device 942, and a DSP (digital signal processor) 944. The programmable logic device 942 may be a FPGA such as the XC3S250E-4TQG144C device from Xilinx Inc., San Jose, Calif. The DSP 944 may be TMS320F2812PGFA device from Texas Instruments, Dallas, Tex.

The receiver 904 receives the signal as further modulated by the tags (provided by the directional coupler 906) and two versions of the 13.56 MHz carrier frequency (one shifted 90 degrees). The receiver 904 then demodulates the tag ID information and provides serial data to the motherboard 710.

Reader Antenna Details

FIG. 9 is a block diagram of the antenna 704 a according to an embodiment of the present invention. The antenna 704 a includes a connector 1002, an optional switch 1004, a matching circuit 1006, an antenna loop 1008, and a shorting loop 1010. These components may be implemented on a two-sided FR-4 printed circuit board assembly.

The connector 1002 connects the antenna 704 a to the daughter board 708. According to an embodiment, the connection to the daughter board 708 is a shielded coaxial cable, and the connector 1002 is a SMA (SubMiniature version A) connector.

The optional switch 1004 can be used to short out the antenna loop 1008 when the antenna loop 1008 is not in use. The switch 1004 is controlled by a DC bias on the antenna cable that may be generated by the daughter card. The switch 1004 acts as an additional shorting loop to reduce the field strength in this antenna's betting spot when an adjacent betting spot is being actively driven. This may improve discrimination between tokens in adjacent betting spots.

The matching circuit 1006 may be a 50 Ohm impedance matching circuit.

The antenna loop 1008 may be a loop antenna. The antenna loop 1008 may be in various form factors according to the specifics of the desired betting area and the desired performance. According to an embodiment, the antenna loop 1008 may be in the form of a rectangle sized at 2 inches by 4 inches. According to an embodiment, the antenna loop 1008 may be in the form of a circle with a diameter of 4 inches.

Gaming Token Details

FIG. 10A is a top view (cut away), FIG. 10B is a bottom view (cut away), and FIG. 11 is a block diagram, of a token 1100 according to an embodiment of the present invention. The token 1100 generally includes a ferrite core, an antenna, and tag electronics. More specifically, the token 1100 includes a printed circuit board 1102, an antenna 1104, the ferrite core 120, a plastic housing 1110, a bridge rectifier 1112, a current source 1114, a shunt voltage regulator 1116, a receive filter 1118, a microprocessor 1120, and a transmit transistor 1122.

The token 1100 may be a gaming token such as is suitable for use in casinos. The token 1100 may be circular with a diameter of 1.55 inches (39.4 mm) and a thickness of 0.125 inches (3.18 mm). These parameters may be varied as desired.

The printed circuit board 1102 may be generally circular in shape, in order to conform to the form factor of circular gaming tokens. The printed circuit board 1102 may be of FR-4 material and 0.020 inches in thickness.

The antenna 1104 may be an 8-turn antenna etched on one side (e.g., the bottom) of the circuit board 1102. These antennas may be constructed with 8 mil traces and 7 mil spacing. The inductance of the 8 turn antenna may be 3 uH. Antennas with different numbers of turns may be implemented with a different balance between inductance and resistance, according to design needs.

Many existing RFID tags use a diode rectifier followed by a voltage clamp to limit the required operating voltage range of the tag and thereby protect the tag from over-voltage. In this embodiment, the power supply may be a linear power supply where there is a bridge rectifier 1112 followed by a current source 1114 and then a voltage clamp (shunt voltage regulator) 1116. This architecture does not clamp the voltage across the coil as is typically done in RFID tags. This linear supply allows tags to operate over a broad range of magnetic field intensities. This allows tags to be read on the top of the stack—where the field is lowest—and near the bottom of the stack—where the field is highest. The net effect is an increased read range. The linear power supply may be as described in U.S. Provisional Application No. 61/031,270 for “Dynamic Power Absorption of a Loop Antenna for Passive RFID Tags” filed Feb. 25, 2008.

The microprocessor 1120 may be a PIC microcontroller from Microchip Technology Inc., Chandler, Ariz. The microprocessor 1120 stores the ID of the token 1100, decodes commands from the reader, and encodes the ID of the token 1100 onto the 13.56 MHz carrier. The microprocessor 1120 includes an internal comparator that is coupled to the receive filter 1118.

The ferrite core 120 may be as described above (see, for example, FIG. 1B). The ferrite core 120 harnesses the H-field and steers it through the antenna 1104. The primary physical attribute of the ferrite is its permeability. Any material that is magnetically permeable and effectively steers the H-field through the antenna loop will improve performance. Ferrite devices are commonly used with frequencies up to 1 GHz. According to an embodiment, the ferrite core 120 has a permeability of 125, plus or minus 20% (e.g., between 100 and 150).

According to an embodiment, the ferrite core 120 is made of a high frequency perminvar NiZn ferrite with a range of inductive applications up to 25 MHz with low losses. A suitable material is the “M” Material from National Magnetics Group, Inc., Bethlehem, Pa. According to an embodiment, the ferrite core 120 is a molded plastic material with a ferrous filler or a ferrous additive. Other materials that may be used are powdered iron, ground up ferrite, soft iron, or a nickel-iron-molybdenum alloy (e.g., molypermalloy), although they may be generally better suited to frequencies lower than 13.56 MHz.

According to an embodiment, the frequency of 13.56 MHz may be used. Different frequencies may be used in other embodiments.

The ferrite core 120 has a thickness that generally conforms to the form factor of the token 1100. For example, the ferrite core 120 may have a thickness of 3.175 mm.

The ferrite core 120 has a diameter that generally conforms to the form factor of the token 1100 and the other internal components. For example, the ferrite core 120 may have a diameter of 12.7 mm. This diameter allows tokens to be mis-aligned in a stack and provide sufficient overlap between adjacent tokens such that flux up the stack is not degraded (i.e., performance is not sensitive to how well the stack is aligned). Practical ranges for the diameter of the ferrite in standard (round) gaming tokens can be as small as 2 mm and as large as 35 mm, depending on other physical requirements. Other form factors such as the use of “plaques” may allow greater freedom in the choice and location of the ferrite element(s). For example, a rectangular plaque may include two circular ferrite cores 120.

The size of the ferrite core may be increased or decreased according to the form factor of the token 1100 and the desired performance characteristics. As long as it fits, there is no reason one could not make the ferrite core 120 bigger in diameter. As long as there is sufficient overlap to not degrade flux up the stack, one could make the ferrite smaller in diameter. There does not appear to be much sensitivity to space between antenna windings and the outside diameter of the ferrite.

The ferrite core 120 may be circular in shape but any shape that fits inside the antenna loop is acceptable. The circular shape was chosen for symmetry and ease of manufacture. The ferrite core 120 may be positioned at locations other than the center of the token 1100 if desired.

According to an embodiment, the diameter of the ferrite core 120 may be increased up to the diameter of the token. The antenna may be wrapped around an outer edge of the token.

The top and bottom of the token 1100 may be covered with labels or stickers (not shown) to denote the denomination or other desirable information. The label may have a thickness of approximately 0.003 inches. This thickness minimizes any air gaps when the tokens are stacked (that is, it helps the ferrite cores in a stack of tokens to function as if they were a single monolithic rod of ferrite). The thickness of the label may be varied, as desired, with corresponding effects on the read performance. Alternately, the ferrite core may be exposed by using an annular sticker to further minimize any air gaps in the stack. The gap performance of between two tokens 1100 is given in TABLE 3, according to an embodiment.

TABLE 3 Gap 0% 1.75% 2.5% 4.5% 9% 17.5% 23.5% Gain (dB) 24 19 15.5 13 10 7 5.5

The “gap” refers to the space between ferrite cores 120, as a percentage of the thickness of each. (For example, a gap of 20% for a thickness of 3 mm corresponds to a 0.6 mm gap.) The gap may result from a label as described above, from a more robust covering for the token 1100, from debossed features on the token 1100, etc. The “gain” refers to the increased signal strength for tokens 1100 having the ferrite core 120 as compared to a token lacking the ferrite core 120. Thus, TABLE 3 shows that for a gap between 0% and 1.75%, the gain is between 24 dB and 19 dB. The information in TABLE 3 is dimensionless; that is, it does not depend upon the number of tokens 1100 in a stack.

The ferrite cores 120 can be manufactured either by cutting them from a solid rod or by sintering them in a special tool. Sintering allows the addition of aesthetic elements to be de-bossed on either one or both facets of the ferrite with little degradation in their performance. Furthermore, sintering allows the designer to add features to allow insert molding of the ferrites during the molding of the tokens.

According to an embodiment, ferrous material or small ferrite beads may be added to a plastic matrix, which is then used to mold part or all of the token 1100. In such an embodiment, the ferrite core 120 and the token 1100 refer to the same structure, and a separate element for the ferrite core 120 is not required.

According to an embodiment, more than one ferrite core may be in a token. According to an embodiment, a token may include more than one RFID tag (e.g., the tag electronics may provide multiple tag IDs for a token). According to an embodiment, more than one antenna may be in a token.

Shorting Loop Details

FIG. 13 shows three views of a loop antenna 1304 and a shorting loop 1306 according to an embodiment of the present invention. The top view shows a three-dimensional view of the loop antenna 1304 and the shorting loop 1306. The middle view shows a cut away side view along Section B-B of the top view, including flux lines 1300. The bottom view shows a graph of the field strength 1302 according to distance from the loop antenna 1304, including a null 1310. A distance “d” separates the loop antenna 1304 and the shorting loop 1306. A “spot” 1320 corresponds to an area interior to the loop antenna 1304 where the field strength 1302 is high. A “border” 1322 corresponds to an area outside the spot 1320 where the field strength 1302 is present except for the null 1310. Outside the spot 1320 and the border 1322, there is no field strength corresponding to the loop antenna 1304.

Comparing FIG. 13 to FIG. 12, note that the tail 1212 is significantly constrained. The shorting loop 1306 distorts the h-field generated by the antenna 1304 to constrain the tail (that would otherwise be present). Note that the particular shape of the field strength 1302 graph will depend upon the specifics of the overall reader system (e.g., the specifics of the loop antenna 1304, the shorting loop 1306, the distance “d”, etc.).

The shorting loop 1306 balances sensitivity (the ability to read RFID tags inside the antenna loop) with cross-talk (the ability to discriminate against RFID tags outside the antenna loop). FIG. 13 shows how the shorting loop 1306 is used to tune the location of the null 1310 and constrain the shape of the tail to a finite and manageable border 1322. From the perspective of designing a gaming table with clearly defined betting zones, the space “d” between the antenna and the shorting loop has both positive and negative attributes. On the positive side, a smaller gap “d” results in a tighter border around the betting zone and reduces cross-talk. On the negative side, a smaller gap “d” will cause the field to collapse-reducing overall sensitivity within the betting zone and thereby reducing the read range (i.e. how high one can stack the gaming tokens and still get accurate data). The gap “d” may be a constant distance between the shorting loop 1306 and the loop antenna 1304. The gap “d” may be a variable gap according to other embodiments, as desired, for example to fine-tune the field shape or for other purposes.

FIG. 14 shows three views of a loop antenna 1404 and a shorting loop 1406 according to an embodiment of the present invention. The top view shows a three-dimensional view of the loop antenna 1404, the shorting loop 1406, and a stack of RFID tokens 1430. The middle view shows a cut away side view along Section C-C of the top view, including flux lines 1400. The bottom view shows a graph of the field strength 1402 according to distance from the loop antenna 1404. The stack of RFID tokens 1430 may include ferrite cores as discussed above.

Comparing FIG. 14 to FIG. 13, note that the flux lines 1400 are distorted (as compared to the flux lines 1300) by the presence of the tokens 1430. The ferrite cores steer the magnetic flux up the stack of RFID tags inside the betting zone border with the active antenna. Tags outside the border do not benefit from this coupling. This difference results in the ability to discriminate between tokens inside a defined border and tokens outside the border. The greater this ability to discriminate, the more well-defined the border. Further note that the field strength 1402 is increased (as compared to the field strength 1302) by the presence of the tokens 1430.

The embodiment of FIG. 13 or the embodiment of FIG. 14 may be incorporated with other embodiments shown in other figures, for example, FIG. 1A, FIG. 6, or FIG. 9.

In summary, the shorting loop 1306 can be placed around the excitation antenna 1304 in a manner that defines the border 1322 around a betting spot 1320 while retaining sufficient sensitivity inside the betting spot 1320. A noteworthy design factor is the comparison between the signal strength of the top tag of a stack of tokens 1430 placed inside the border 1322 with the signal strength of a bottom tag outside the border 1322. The ability to differentiate between the two (along with the design of the circuit) may help determine the width of the border 1322 around a betting spot 1320.

According to an embodiment, an excitation power of 1 Watt drives the antenna 1304. The shorting loop 1306 may be located outside of the antenna 1304 with a distance “d” of 0.5 inches. The shorting loop 1306 may be located outside of the antenna 1304 with a distance “d” of 0.125 inches. Note that placing the shorting loop 1306 too close to the excitation antenna 1304 has a negative impact on signal strength.

According to an embodiment, the gaming table 700 (see FIG. 6) includes a covering over the antennas. The covering may have visual indicators of the betting areas. The covering may include felt or padding. The thickness of the covering may be referred to as the “table gap”. Various table gaps may be used in various embodiments, including 0 inches (no covering), 0.25 inches, 0.5 inches, and 0.75 inches.

According to an embodiment, the antenna 1304 may have a trace width of 0.25 inches. According to an embodiment, the shorting loop may have a trace width of 0.25 inches. According to an embodiment, the shorting loop may be formed as a copper trace on the printed circuit board that also contains the excitation antenna. Other materials may be used in other embodiments. The shorting loop trace may be approximately 0.125 inches wide and separated from the excitation antenna by 0.25 inches (i.e., the center of the antenna loop trace is 0.5 inches from the center of the shorting loop trace). Other dimensions may also be implemented in other embodiments. As noted above, the gap between antenna and shorting loop is a tradeoff between the desire to extend the read range (bigger gap) and the desire to sharply define the border of the betting zone (smaller gap). The extreme case of needing a sharply defined border is when multiple antennas abut each other.

FIG. 15 is a flow diagram of a method 1500 of performing RFID according to an embodiment of the present invention. The method 1500 may be performed by a RFID system that includes one or more of the embodiments discussed above.

In step 1510, a shorting loop is provided. The shorting loop at least partially surrounds an RFID antenna. The shorting loop may be the shorting loop 1306.

In step 1520, the antenna generates an electromagnetic field. The electromagnetic field includes a magnetic field component. The electromagnetic field may include a modulated signal for communicating information to and from RFID tags.

In step 1530, the shorting loop distorts the magnetic field component to improve a definition of a border of a read region related to the antenna. In a gaming embodiment, the read region corresponds to a betting spot, and the shorting loop distorts the magnetic field component such that tokens outside of the betting spot do not receive (or receive only a small amount of) the magnetic field component.

In step 1540, the RFID system reads an RFID tag in the read region. More generally, the RFID system detects whether the RFID tag is inside or outside of the read region, and it is desired to read only those tags inside of the read region. The tags outside the read region are either not detected, or if they are detected they are recognized as being outside the betting spot (due to a low detected signal strength or a high signal strength differential compared to other detected tokens). The reading can further include further details as discussed above.

One performance measurement that may be used when evaluating embodiments of the present invention is the “10 dB discrimination distance”. The 10 dB discrimination distance is a metric that defines a distance from the border of a defined read region at which point the signal strength from an RFID tag outside this distance is at least 10 dB less than the weakest signal from any RFID tag inside the read region. For example, in a gaming table embodiment, a token outside the betting zone has a signal strength at least 10 dB less than the weakest signal from any token inside the betting zone. The tokens inside the betting zone may be stacked (e.g., 30 tokens stacked) and the tokens outside the betting zone may be stacked adjacent to the inside tokens, and the 10 dB discrimination distance criterion is met according to an embodiment of the present invention.

The 10 dB discrimination distance is a non-dimensional ratio that is independent of absolute signal strength and antenna geometry. The 10 dB discrimination distance may be used to measure RFID systems without the shorting loop, for comparison purposes. As an example, the 10 dB discrimination distance for an antenna without a shorting loop is 3 or more inches; using the same antenna with a shorting loop, the 10 dB discrimination distance is less than zero (that is, tokens inside that are adjacent to tokens outside result in more than 10 dB difference in signal strength).

The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents will be evident to those skilled in the art and may be employed without departing from the spirit and scope of the invention as defined by the claims. 

1. A radio frequency identification (RFID) system, comprising: an antenna that generates an electromagnetic field that includes a magnetic field component; and a shorting loop that at least partially surrounds the antenna, wherein the shorting loop distorts the magnetic field component to improve a definition of a border of a read region related to the antenna.
 2. The RFID system of claim 1, wherein the shorting loop is displaced from the antenna by a gap, wherein a size of the gap is selected to balance a read range and a discrimination factor.
 3. The RFID system of claim 2, wherein the discrimination factor corresponds to a signal strength difference between a first RFID tag inside the read region and a second RFID tag outside the read region.
 4. The RFID system of claim 2, wherein the gap is a constant distance between the shorting loop and the antenna.
 5. The RFID system of claim 2, wherein the gap is a variable gap.
 6. The RFID system of claim 1, further comprising: an RFID reader, coupled to the antenna, that provides power to the antenna to generate the electromagnetic field, that provides a first modulated signal to the antenna, and that receives a second modulated signal from the antenna.
 7. The RFID system of claim 1, further comprising: an RFID reader, coupled to the antenna, that includes for reading a first RFID tag inside the read region and that excludes from reading a second RFID tag outside the read region.
 8. The RFID system of claim 1, wherein the antenna is a circular antenna, wherein the shorting loop is a circular shorting loop, and wherein the circular shorting loop surrounds the circular antenna.
 9. The RFID system of claim 1, wherein the antenna is a rectangular antenna, wherein the shorting loop is a rectangular shorting loop, and wherein the rectangular shorting loop surrounds the rectangular antenna.
 10. The RFID system of claim 1, wherein the shorting loop is made of a copper material.
 11. The RFID system of claim 1, wherein the shorting loop constrains the electromagnetic field outside of the read region.
 12. The RFID system of claim 1, wherein a first signal strength of a first RFID tag within the border is more than 10 dB greater than a second signal strength of a second RFID tag that is outside the border and adjacent to the first RFID tag.
 13. The RFID system of claim 1, further comprising: a plurality of RFID tokens, each RFID token including a magnetically permeable core that distorts the magnetic field component and, in accordance therewith, improves a field strength of the magnetic field component within the read region.
 14. A method of performing radio frequency identification (RFID), comprising: providing a shorting loop that at least partially surrounds an antenna; generating, with the antenna, an electromagnetic field that includes a magnetic field component; distorting, with the shorting loop, the magnetic field component to improve a definition of a border of a read region related to the antenna; and reading an RFID tag in the read region.
 15. A radio frequency identification (RFID) system, comprising: a plurality of antennas that each generate an electromagnetic field that includes a magnetic field component, wherein the plurality of antennas includes a first antenna and a second antenna that is near the first antenna; and a plurality of shorting loops, wherein each of the plurality of shorting loops at least partially surrounds a corresponding one of the plurality of antennas, wherein a first shorting loop of the plurality of shorting loops distorts the magnetic field component of the first antenna to reduce interference with the magnetic field component of the second antenna. 